This invention relates to a GaN semiconductor element used as a light source, for example, of an indicating lamp, full color display or DVD (Digital Versatile Disc)system, and its manufacturing method.
GaN and other nitride semiconductors are practically useful as semiconductors to make blue-color diodes, blue-color laser diodes, and so forth. In.sub.x Ga.sub.1-x N, among them, can be changed in band gap in the range from 2 eV to 3.4 eV by adjusting the mole fraction x of In, and is especially remarked as a hopeful material of light emitting elements for light in the visible band.
In the present application, "nitride semiconductors" involve III-V compound semiconductors expressed by B.sub.x In.sub.y Al.sub.z Ga.sub.(1-x-y-z) N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, 0.ltoreq.z.ltoreq.1), and involves mixed crystals including phosphorus (P) or arsenic (As) in addition to N as the group V element.
Double-heterostructures using an In.sub.x Ga.sub.1-x N layer, such as structures sandwiching an In.sub.x Ga.sub.1-x N active layer between AlGaN cladding layers, are effective for confinement of injected carriers and light, and are employed to made light emitting elements for a high luminance or a short wavelength.
Explained below are a semiconductor light emitting element with a double-heterostructure using an InGaN layer as its active layer and a method for manufacturing same.
FIG. 12 is a cross-sectional view schematically showing the semiconductor light emitting element of a double-heterostructure including a InGaN active layer and AlGaN cladding layers. FIGS. 13A through 13D are cross-sectional views schematically showing the semiconductor light emitting element of FIG. 12 under different steps of its manufacturing process.
As shown in FIG. 12, sequentially stacked on a sapphire substrate 21 are a GaN buffer layer 22, n-type Al.sub.y Ga.sub.1-x N cladding layer 23 (0.ltoreq.y.ltoreq.1), In.sub.x Ga.sub.1-x N active layer 25 (0.ltoreq.x.ltoreq.1), p-type Al.sub.z Ga.sub.1-z N cladding layer 27 (0.ltoreq.z.ltoreq.1) and p-type GaN layer 28 which are stacked sequentially on a sapphire substrate 21.
These layers are typically grown by MOCVD (Metal-Organic Chemical Vapor Deposition) in the following process. In the explanation made below, the sapphire substrate 21 is named substrate 21, and mole fractions x, y and z of layers are omitted.
The substrate 21 is first introduced into a MOCVD reactor, and annealed in a flow of hydrogen gas at 1100.degree. C. for 10 minutes. After that, the temperature of substrate 21 is decreased to 520.degree. C., and the GaN buffer layer 22 is grown to 50 nm on the surface of the substrate 21 (see FIG. 13A).
Then, the substrate 21 is heated to 1100.degree. C., and the n-type AlGaN cladding layer 23 is grown to 4 .mu.m, maintaining the temperature at 1100.degree. C. (see FIG. 13B).
Thereafter, the temperature of the substrate 21 is decreased to 750.degree. C., and the InGaN active layer 25 is grown to 0.1 .mu.m, maintaining the temperature at 750.degree. C. (see FIG. 13C).
Then, the substrate 21 is heated to 1100.degree. C., and the p-type AlGaN cladding layer 27 of 0.15 .mu.m and the p-type GaN contact layer 28 of 0.3 .mu.m are grown under the constant temperature of 1100.degree. C. Thus the light emitting element with a double-heterostructure is formed (see FIG. 13D).
For growth of the cladding layer 23 and 27, the temperature must be set at about 1000.degree. C. which is higher by 200 through 350.degree. C. than the growth temperature for the InGaN active layer 25. That is, before and after the growth of the InGaN active layer 25, the temperature of the substrate 21 had to be increased and decreased. Because of GaN having the melting temperature of about 1000.degree. C. and InN having the melting point of about 500.degree. C., the process involved the following problems.
1. Decomposition of InN occurs in the InGaN active layer 25 in the process of increasing the temperature. GaN, however, is very unlikely to be decomposed. Therefore, the InGaN active layer 25 is partly replaced by GaN, and its crystallographic property deteriorates.
2. Decomposition of InN in InGaN in the process of increasing the temperature results in decreasing the thickness of the InGaN active layer 25.
3. The hetero-interface between the cladding layer 23 and the InGaN active layer 25 degrades due to the problems 1 and 2 indicated above in the process of increasing the temperature.
4. In the process of decreasing the temperature, an unintentional InGaN layer not having the predetermined mole fraction of In may be grown, and the hetero-interface between the active layer 25 and the cladding layer 27 may degrade.
Deterioration of the crystallographic property of the active layer 25 and degradation of the hetero-interface invite a decrease in emission efficiency of light emitting elements and an increase in threshold value of laser elements. Further, the decrease in thickness of the InGaN active layer 25 means a structural deviation of the element from a designed value, and it invited degradation of initial characteristics and reliability of light emitting elements.